On Thu 13 Apr 2017 05:17:21 PM CEST, Denis V. Lunev wrote: > On 04/13/2017 06:04 PM, Alberto Garcia wrote: >> On Thu 13 Apr 2017 03:30:43 PM CEST, Denis V. Lunev wrote: >>> Yes, block size should be increased. I perfectly in agreement with >>> your. But I think that we could do that by plain increase of the >>> cluster size without any further dances. Sub-clusters as sub-clusters >>> will help if we are able to avoid COW. With COW I do not see much >>> difference. >> I'm trying to summarize your position, tell me if I got everything >> correctly: >> >> 1. We should try to reduce data fragmentation on the qcow2 file, >> because it will have a long term effect on the I/O performance (as >> opposed to an effect on the initial operations on the empty image). > yes > >> 2. The way to do that is to increase the cluster size (to 1MB or >> more). > yes > >> 3. Benefit: increasing the cluster size also decreases the amount of >> metadata (L2 and refcount). > yes > >> 4. Problem: L2 tables become too big and fill up the cache more >> easily. To solve this the cache code should do partial reads >> instead of complete L2 clusters. > yes. We can read full cluster as originally if L2 cache is empty. > >> 5. Problem: larger cluster sizes also mean more data to copy when >> there's a COW. To solve this the COW code should be modified so it >> goes from 5 OPs (read head, write head, read tail, write tail, >> write data) to 2 OPs (read cluster, write modified cluster). > yes, with small tweak if head and tail are in different clusters. In > this case we > will end up with 3 OPs. > >> 6. Having subclusters adds incompatible changes to the file format, >> and they offer no benefit after allocation. > yes > >> 7. Subclusters are only really useful if they match the guest fs block >> size (because you would avoid doing COW on allocation). Otherwise >> the only thing that you get is a faster COW (because you move less >> data), but the improvement is not dramatic and it's better if we do >> what's proposed in point 5. > yes > >> 8. Even if the subcluster size matches the guest block size, you'll >> get very fast initial allocation but also more chances to end up >> with a very fragmented qcow2 image, which is worse in the long run. > yes > >> 9. Problem: larger clusters make a less efficient use of disk space, >> but that's a drawback you're fine with considering all of the >> above. > yes > >> Is that a fair summary of what you're trying to say? Anything else >> missing? > yes. > > 5a. Problem: initial cluster allocation without COW. Could be made > cluster-size agnostic with the help of fallocate() call. Big > clusters are even > better as the amount of such allocations is reduced. > > Thank you very much for this cool summary! I am too tongue-tied.
Hi Denis, I don't have the have data to verify all your claims here, but in general what you say makes sense. Although I'm not sure if I agree with everything (especially on whether any of this applies to SSD drives at all) it seems that we all agree that the COW algorithm can be improved, so perhaps I should start by taking a look at that. Regards, Berto
